This application relates to optical fibers and fiber devices with one or more fibers engaged on substrates.
Optical fibers can be used to transmit or process light in a variety of applications, including, among others, delivering light to or receiving light from integrated optical components or devices formed on substrates, transmitting information channels in wavelength-division multiplexed optical communication devices and systems, forming fiber optic switch matrix devices or fiber array to array connector, producing optical gain for optical amplification or laser oscillation, and modulating guided light. One of the features of the optical fibers in those and other different applications is that an optical fiber operates as xe2x80x9ca light pipexe2x80x9d to transport optical energy. A typical fiber may be simplified as a fiber core and a cladding layer surrounding the fiber core. The refractive index of the fiber core is higher than that of the fiber cladding to confine the light. Light rays that are coupled into the fiber. core within a maximum angle with respect to the axis of the fiber core are totally reflected at the interface of the fiber core and the cladding. This total internal reflection provides a mechanism to spatially confine the optical energy of the light rays in one or more selected fiber modes to guide the optical energy along the fiber core.
The guided optical energy in the fiber, however, is not completely confined within the core of the fiber or waveguide. A portion of the optical energy can xe2x80x9cleakxe2x80x9d through the interface between the fiber core and the cladding via an evanescent field that essentially decays exponentially with the distance from the core-cladding interface. The distance for evanescent decay in the electric field of the guided light by a factor of 2.718 is about one wavelength of the guided optical energy. This evanescent leakage may be used to couple optical energy into or out of the fiber core, or alternatively, to perturb the guided optical energy in the fiber core.
The present disclosure includes structures that integrate one or more fibers to a substrate by, e.g., bonding the fiber to the substrate. In general, the material properties of the substrate may be different from those of the fiber material, e.g., the coefficient of thermal expansion. Hence, when the fiber is directly engaged to the substrate, the fiber and the substrate respond differently to environmental changes such as temperature and other factors such as aging. As a result, such fiber structure may be relatively unstable.
The devices and techniques of the present disclosure include one or more thin-film buffer layers positioned between the fiber and the substrate to provide a transition structure whose certain material properties such as the coefficient of thermal expansion have values between those of the fiber and those of the substrate. Hence, the overall stress due to the material mismatch in the buffered device is reduced compared to the stress in the non-buffered device where the fiber is directly engaged to the substrate.
The material of the buffer layer may also be selected to allow for diffusion between the buffer layer and the fiber at their contact locations. Each contact location may be locally heated to promote diffusion so that a direct diffusion bond can be formed to bond the fiber to the buffer layer.
In addition, a thin film formed over the substrate surface may also be used as a thickness-monitoring element for monitoring removal of the fiber cladding of a fiber that is engaged to an elongated groove on the substrate. In particular, this thin-film thickness-monitoring element can operate in sito to provide real-time information while the fiber cladding is being removed.
Embodiments of the invention include the following techniques.
In one embodiment, a method of this application includes the following operations:
forming an elongated groove over a substrate surface of a substrate;
forming a monitoring layer over at least said substrate surface adjacent to said elongated groove;
placing a fiber in said elongated groove to protrude a portion of fiber cladding above the initial surface of said monitoring layer;
bonding said fiber to said elongated groove;
removing said fiber cladding and said monitoring layer to be substantially coplanar with each other; and
monitoring a thickness of said monitoring layer to determine whether a desired amount of fiber cladding is removed.
In the above method, the monitoring step may be performed during said removing step.
The above monitoring step may also be performed by measuring a reflected beam from reflecting an optical probe beam off said monitoring layer. This measurement of the reflected beam may be carried out in a number of ways, such as measuring interference in said reflected beam, measuring optical attenuation in said reflected beam by absorption of said monitoring layer, an ellipsometry measurement, and measuring a color shifting with respect to an incident angle of said probe beam.
The above method may also include:
forming a witness window in said monitoring layer to expose said substrate surface; and
measuring a difference in height between a top surface of said monitoring layer and said substrate surface exposed in said witness window in monitoring said thickness of said monitoring layer.
In implementations, the monitoring layer may include a stack of two alternating films of different sheet resistance values and substantially identical thickness. With this film stack, the method may further include the following additional operations:
measuring a sheet resistance to identify which film is exposed; and
determining said thickness of said monitoring layer by a number of films remaining over said substrate surface.
In another embodiment, a method of this application may include:
forming an elongated groove over a substrate surface of a substrate;
forming a buffer layer over surfaces of said elongated groove and at least the substrate surface adjacent to said elongated groove;
placing a fiber in said elongated groove to protrude a portion of fiber cladding above the initial surface of said buffer layer that is over said substrate surface;
bonding said fiber to said buffer layer in said elongated groove;
removing said fiber cladding and said buffer layer over said substrate surface to be substantially coplanar with each other; and
monitoring a thickness of said buffer layer over said substrate surface to determine whether a desired amount of fiber cladding is removed,
wherein a material of said buffer layer is different from said fiber and said substrate.
The monitoring step may be performed during said removing step and may be performed by measuring a reflected beam from reflecting an optical probe beam that illuminates said buffer layer over said substrate surface.
In one implementation, a material of said buffer layer is selected to permit material diffusion with said fiber, and wherein said bonding is achieved by locally heating a contact location between said fiber and said buffer layer in said elongated groove.
The bonding step may include a use of an adhesive to engage said fiber to said buffer layer in said elongated groove, or using CO2-assisted welding to engage said fiber to said buffer layer in said elongated groove. In yet another implementation, the bonding may be achieved by the following additional steps:
applying a liquid gel between said fiber and said buffer layer in said elongated groove; and
heating up said liquid gel to a solidified state to bond said fiber to said buffer layer in said elongated groove.
The material of said buffer layer may be selected to have a material parameter with a value that is between a first value of said material parameter for said substrate and a second value of said material parameter for said fiber. This material parameter may be a coefficient of thermal expansion. Furthermore, the buffer layer is a liquid gel which solidifies when heated to a temperature, and the bonding in this situation includes heating up said butter layer to said temperature.